(1) Field of the Invention
The present invention relates generally to systems for bubble control, and more particularly to a system for coalescing bubbles in the wake of a structure piercing the surface of the water.
(2) Description of the Prior Art
A moving structure interacting with a free liquid surface produces a surface wake. The complicated fluid flow around a moving structure entrains air bubbles into the flow which ultimately enter the surface wake. The presence of bubbles in the surface wake strongly impact the acoustical properties of the wake. In many instances, it is desirable for surface piercing structures (e.g., surface ships, towed arrays, submarine masts, etc.) to be able to control the acoustical properties of the wakes they generate. One way to manipulate these properties is to control bubble size in the wake.
Bubbles in a wake can be detected acoustically. Accordingly, the longer bubbles are present in a wake, the longer the wake serves as a means of detection. The amount of time that bubbles remain in a wake is primarily a function of the bubble size. Large bubbles rise quickly to the water's surface where they diffuse in the air. However, small bubbles rise much slower. Small bubbles are also more likely to be drawn into persistent large scale vortices. One way to reduce or eliminate bubbles from a wake is to coalesce small bubbles into larger bubbles before they diffuse over a wide area.
Bubbles observed in breaking waves in seawater are on the order of 200 microns in size. Bubbles observed in breaking waves in fresh water are on average 1500 microns in size. Empirical evidence shows that persistent bubbles in fresh water are rare whereas they are common in sea water. Thus, if a cloud of 200 micron bubbles could be changed into a cloud of 1500 micron bubbles relatively quickly, the larger size bubbles could quickly rise to the water's surface and diffuse in air. However, at present, there are no effective means for coalescing small bubbles in a wake to form larger bubbles which will quickly rise to the surface.
U.S. Pat. No. 5,613,456 teaches a microbubble positioning and control system that positions bubbles in a fluid flow. Two spaced-apart acoustic sources are angled towards one another. Acoustic plane waves of differing frequency generated by the acoustic sources intersect to form a standing wave field that sweeps the bubbles in a desired direction. However, this system does not teach a system or method for coalescing small bubbles in a wake to form large bubbles which will quickly rise to the water's surface.